What is Fault Move ment? what is Dipslip Movement?

Dear reader we have already discussed about movement of fault in our previous post. Here actually we will define type of fault movement. Fault movement can geologically reduced to two components i.e. in strike and dip direction. The movements are:

a. Dip slip movement

b. Strike slip movement

But some movements have components in both directions; however only one direction of movement is generally predominant.

 

In this post We will discuss about dip-slip fault movement. From the term dip-slip movement, we can easily realize that movement is primarily occurred along dip direction or can be said normal to strike. Different types of dip-slip movement also found depending on dip angle and direction of movement of respective fault. They are:

a. Normal faults

b.Reverse fault

c.Thrust faults

a. Normal faults:

These types of movement are happened when dip-slip movement have horizontal component of extensional type. in this regard we are introducing two term

-Hang wall

-Foot wall

Hang wall:

The materials remain above inclined fault plane is called hang wall as shown in figure below:

Foot wall:

The materials beneath fault plane is called foot wall as shown in figure above.

So when hang wall moves down ward with respect to foot wall, normal fault movement is occurred which is usually observed in conjunction with tensile stress within the earth crust producing lengthening of crust in horizontal direction.

b.Reverse faulting:

When the movement have compressional horizontal movement, reverse faults are said to be occurred. In this movement hang wall move upward with respect to foot wall and horizontal shortening (crust) is observed.

c. Thrust faulting:

In this movement also hang wall move upward with respect to foot wall but difference is small angle of dip of fault plane. The example is European Alps which is thrust structure indicating potential of producing large movement.

Grain Size of Foundation Soil Leading Sliding Failure of Dam

Dear reader in our last post we have discussed about existence clay soil having high plasticity in foundation and embankment itself and their contribution in sliding type dam failure. Our discussion, here, is also related to this topic; but we will discuss this relation to grain size.

A study was conducted over 65 dams in western part of United States; most of them were old homogeneous dams. The D₅₀values of soils that were used on construction of these dams in essentially their impervious section were collected to make a correlation with grain size and sliding failure.

The D50 values were found ranging from very finer like as fine as less than 0.005mm to coarser materials of greater than 2.0 mm. Here we are providing a figure depicting relationship between average grain size of soil and occurrence of sliding.

Except grain size, other factors that are considered in determining sliding potential are:

a. Steepness of slope

b. Construction method

c. Reservoir activity

The factors stated above are outweighed by influence of grain size. From these studies it can be concluded:

a. All embankments constructed with clay having average value of grain size (D₅₀) finer than 0.006 mm, suffer sliding failure.

b. Half of all embankments constructed with clay having average value of grain size (D₅₀) ranging from 0.02 mm to 0.006 mm (approximately), suffer sliding failure.

c. Small fraction of embankments constructed with soils having average value of grain size (D₅₀) ranging from 0.02 mm to 0.06 mm, suffer sliding failure.

d. None of embankments constructed with soils having average value of grain size (D₅₀) coarser than 0.06 mm in their foundation suffer sliding failure.

Even though there have steep slopes and worst combination of poor compaction.

Rainfall and Settlement Behavior of Foundation Soil, Loess

Whatever the internal structures loess soil show more or less collapse nature and threaten foundation safety. We know consolidation is associated with settlement. In this post we will learn about typical bearing capacity of loess in relation to dry density and rainfall induced failure of foundation on loess soil.

An annual average rainfall of a region has strong correlation with moisture content. Though water table rises with seasonal change, average moisture content depends on average rainfall for a year. The internal structure of loess such that rain water can do the harm quickly.

Recall the structure of loess soil; it is homogeneous, as usual for wind transported soil, have high void ratio and capillaries traverse in vertical direction which results fractures in sediments and vertical bluffs are formed.

Dear reader we have discussed void ratio and permeability of loess soil in previous posts. We are providing necessary links for convenience.

These loosely arranged structure of loess with numerous voids, allows rainwater to infiltrate quickly and we have known in previous post that moisture content increment alone can collapse in the presence of small consolidation pressure.

We know that horizontal permeability of loess soil is less than vertical permeability which is an exception in soil mechanics. The moisture content of loess soil (in-situ) lies between (4-49)%. Now we are going to summarize some findings about loess that correlates moisture content, density and bearing capacity of this soil.

Bandyopadhyay (1983), Holland and Gibbs (1960):

They provided settlement relation with density:

-Settlement is suspected to be large when dry unit weight is less than 80 pcf

-Settlement is suspected to be small when dry unit weight is greater than 90 pcf

From this finding Bandyopadhyay concluded that such soils that have low in-situ densities and also low clay cementation are considered high consolidation and collapse susceptibility.

A dry loess soil may have bearing capacity more than 10 Ksf which may fall down to 0.5 Ksf under saturation.

Now some points about moisture and density correlation:

Holland and Gibbs:

At moisture content less than 10 percent, dry density reached its maximum value and as discussed above highest resistance against settlement is expected.

Moisture content (10-15) % ---soil shows moderate high strength

Moisture content around 20% ---strength decreased gradually

Moisture content > 20%----this moisture content is considered high for loess soil that leads to full consolidation under load.

Moisture content 35%---soil becomes saturated.